1. Field of the Invention
The present invention relates to a position sensor. It is applicable both to an angular sensor detecting rotation of one body relative to another, and to a linear sensor detecting linear movement.
2. Summary of the Prior Art
A variety of devices exist which are capable of sensing the angular position of a rotating shaft. Of these, one of the most versatile is the optical shaft position encoder, having attached to the shaft a disk carrying a binary scale formed by a track of indicia from which angular position is read by an electro-optical detector disposed along the radius. The accuracy obtainable by such techniques depends on the fineness of the binary scale and the ability of the detector to resolve detail, and it is normally accepted that a division of one complete turn into 65,536(2.sup.16) divisions represents the highest precision which can be achieved within normal dimensions by a purely binary system. Further sub-division, up to a limit of about 2.sup.20 parts in one revolution, can be achieved by adding analogue tracks in which the optical transmission varies with angular rotation in a sine or cosine manner, and using these to interpolate between the finest binary divisions. Performance to these limits of accuracy is obtainable only by using high resolution tracks formed photographically on glass disks with reading heads sensing along a radius a very narrow width of the disk, maintaining very small clearances between the reading head and the disk, and with extremely stringent limits on the eccentricity of the tracks on the disk.
Angular encoders of this type are by their very nature delicate instruments. The shock loads which they can sustain are typically less than would be expected merely from the fragility of the glass disk, since a relatively insignificant flexure of the disk permits it to strike the reading head carrying the detector. A possible solution to these problems might be to read a reflective pattern deposited on a metal disk, but the performance of such an encoder would be limited by the fineness and precision of the reflective pattern which can be formed.
Another technique which has been tried for interpolating between coarse scale divisions is to add to the scale tracks on the disk a pattern of alternate light and dark bars, of which a portion is imaged on a detector having a row of sensing elements in such a way as to generate a form of moire fringe system similar to those developed by the use of optical gratings in combination, which is known in the design of incremental encoders. However, such attempts to use moire fringe techniques for interpolation, as opposed to merely counting grating lines, run into a number of difficulties which are essentially associated with the small difference which has to be maintained between the pitch of the indicia formed by bars in the bar pattern image and the pitch of the sensing elements of which they fall. To achieve a useful degree of interpolation requires extremely accurate control of the optical magnification, in addition to the need to maintain a precise mechanical relationship between the interpolation system and the remainder of the encoder scale reading system. In addition, the necessary small difference between the bar pattern image pitch and the sensing element pitch leads to subdivision by factors which are inconvenient to analyse rather than, for example, a precise binary subdivision. Another problem with attempts to use this type of subdivision is that the reading of the scale prior to subdivision is typically dependent upon sensing individual edge transitions in the scale pattern. Since in many cases a speck of dirt or a hair can introduce errors in the scale reading, there is little point in performing further interpolation.
An example of such a position sensor is disclosed in German Offenlegungsschrift No. 3,150,349 in which the optical image of bar markings are imaged as a scale having a track of indicia on to a detector formed by a linear (one-dimensional) array of light sensitive elements. In order for such a system to operate satisfactorily the optical system which images the markings onto the detector must operate extremely accurately as magnification error, or radial displacement of the image would result in no or an erroneous result from the detector.